If you looked up stingrays on Wikipedia last week, you would have learnt that, as well as living in tropical coastal waters and reproducing in litters of five to 10 offspring, the cartilaginous marine fish also 'hate Australian people'.

It wouldn't take long to realise that the last bit isn't true, or certainly that no one asked stingrays whether it was.

In fact it was a piece of 'internet vandalism' by a growing band of cyberspace guerrillas that is targeting sites such as Wikipedia. Since the death of Steve Irwin - the Australian television naturalist who was struck in the chest by a stingray's barb last year - the entry has become one of the online encyclopaedia's most regularly vandalised articles.

The internet has always been a magnet for bored people looking for amusement, but while some write a blog and others search for pornography, a growing number get their kicks by sabotaging high-profile websites. Any site that relies on well-meaning contributions from the public is vulnerable - MySpace, Slashdot and YouTube have all been victims - but Wikipedia is particularly susceptible because it is the encyclopaedia anyone can edit, including those whose sole aim is to undermine it. And whereas MySpace and YouTube employ centralised moderators, Wikipedia relies on the goodwill of loyal members of the community to weed out malicious contributions.

Wikipedia is one of the great internet successes of the decade. Launched in 2001, it quickly became the world's most popular non-profit website and is now among the top 10 visited on the net, containing more than six million articles in 250 languages. With only seven paid administrative employees, the encyclopaedia depends on the altruistic collaboration of its four million registered users, as well as countless others who edit the site anonymously. It has become very powerful in the world of the internet and is used as a reference tool for everyone from students to politicians.

Many question how an encyclopaedia written on this basis could ever work. Wikipedia has been accused of being unreliable and inconsistent, and in 2005 American journalist John Seigenthaler blasted it as 'flawed and irresponsible' after libellous claims were introduced into his biography. This month it was revealed that a prominent and long-standing editor had lied about his identity to win arguments with other Wikipedians: 'Essjay' claimed to be a tenured university professor on his personal user page, when he was actually a 24-year-old community college drop-out.

Wikipedia now faces a controversy that could undermine its whole philosophy - a community of people free to add and improve on its entries. Vandals want to add, but improve is not in their vocabulary.

There are some obvious targets for the Wiki vandals, such as the articles on George W Bush, Britney Spears, abortion and Islam, as well as more bizarre choices, such as the pages on bigfoot, elephants, bananas and liver. Articles covering current events, such as the death of Anna Nicole Smith, are often sabotaged. The dyslexia article is vandalised daily by anonymous users who enjoy scrambling the text to mess up the spelling.

'The more popular Wikipedia becomes, the more vandals it attracts,' says a spokesman from the Wikimedia Foundation, which runs Wikipedia's administration. 'But the older we get, the more we are developing ways to fight it. We have literally thousands and thousands of volunteers worldwide who are very dedicated to the project and work on combating vandalism every day.'

Theresa Knott is one such devoted Wikipedian. A member since 2001, she visits the site daily, often editing at 5.30am before she leaves for work as a London primary school teacher. Her efforts have been rewarded with regular abuse from vandals and kudos from her Wikipedia peers, who elected her to the position of administrator in 2003. This means she is one of more than 1,000 Wikipedians who have special powers to lock down vulnerable articles from further editing, and temporarily block problem users from making changes to the site - the Wikipedia equivalent of an Asbo.

'I'm sure that most of the people who do it are young,' she says. 'If you check their internet provider addresses, it's often from schools and universities. They write the same kind of things over again: it's always "My friend is gay", "My teacher is gay". It's part of the general anti-authority thing kids have.'

The more damaging edits come from problem editors who register a user name in the hope of gaining online notoriety. Alm93, also known as the Sneaky Stats Vandal, caused havoc last year by making subtle and inaccurate changes to important statistics, such as population figures and average human height.

Another infamous vandal, Willy on Wheels, liked to move pages, so that those looking for the George Bush page would be redirected to another called something like 'George Bush loves to Suck Willy on Wheels'. He regularly tried to do this to hundreds of articles at a time, registering a new user name whenever he was blocked. 'I suspect it wasn't always him,' sighs Knott. 'He became quite famous and I think people copied him.'

The standard way to stop a vandal is to 'revert' their changes and leave a warning on their personal user page. Most then lose interest, but if they persist Knott will block them for a limited period, say 31 hours, or longer if they are a repeat offender. 'We tend to keep blocks short and sweet, and that seems to work.'

Wikipedia is remarkably robust in the face of regular attacks. Most casual users are unaware of the problem because vandalism tends to be spotted very quickly: the comments on the stingray article lasted less than five minutes before another editor changed the page back. Some damaging contributions can be discovered by purpose-made 'bots' - computer programmes designed by users to catch obvious vandalism, such as profanities and producing blank pages. The more insidious edits are spotted by an army of editors who scour the list of recent changes in search of sabotage.

'Vandalism would be difficult to police if there were more vandals, but the ratio of vandal editors to non-vandals is too low,' Knott explains. 'Sometimes I'll go to block a vandal and find someone has beaten me to it, and I'm somewhat annoyed. There are a lot more of us than there are of them, and administrators have a lot more powers than they do. It's not a fair fight at all.'

It's a fight Wikipedia says it is guaranteed to win, because the site constantly adapts to meet new challenges. 'If they come in with new methods, we change the rules. The rules are whatever we say they are and we won't stick to our own rules if it's bad for Wikipedia.' This means that the definition of a vandal is also fluid - it's whatever the administrators think it is at a given time.

This has caused a backlash among some editors, who argue that blocking users compromises the supposedly open nature of the project, and the imbalance of power between users and administrators may even be a reason some users choose to vandalise in the first place.

The self-styled Louisville Vandal, who inserted racist comments into articles, says his work was in retaliation for unjust sanctions: 'My vandalism started after an edit conflict over the Courier-Journal's sports and editorial coverage, where my - what I felt were - legitimate edits on the page for C-J criticism were removed and I was blasted,' he says. 'I have being vandalising Wikipedia and its user pages for months, mostly because seeing my vandalism or that of others was funny as hell... and to punish admins.'

But Knott insists that a good administrator will never be upset by a vandal. She attributes her success to her 'cool, calm and collected demeanour', which never falters - even when she is faced with death threats. Last month an irate vandal emailed her with the words, "So ur the worthless piece of shit that was deleting my edits on the Smallville page... I'll track u down to where you live, u have no idea what trouble ur in. I'm gonna slice ur throat & stab ur eyes out... ur a dead person.' Knott reported him to his internet service provider and carried on working, unfazed.

'It's nothing to worry about,' she says. 'He clearly doesn't mean what he says. He's obnoxious but not scary. Most of the time, the vandals are just having a joke. If you vandalise a Wikipedia page, it's so easy to revert it that it really isn't a big deal. I don't think the world's coming to an end because someone is having a joke. Sometimes I even laugh at them. But then I revert them. Then I tell them that if they do it again, they'll get blocked.'

And so Theresa Knott gets on with her work - protecting one of the most successful websites of the 21st century.

Virtual vandalism

FacebookThe social networking site has been blamed for carrying abuse about students and tutors through the creation of false profiles. Dr Henry Bennet-Clark, of Oxford University, only discovered that he had a Facebook entry when it was picked up by the student newspaper Cherwell. The profile, created by a second-year computer science student, claimed the don had been a member of the Hitler Youth. Bennet-Clark said: 'I, personally, was rather careful to do nothing, but we had some crisp words with the perpetrator and he was summarily disconnected for the rest of that term.'

MySpaceSome users of the leading social networking site have had their pictures 'hijacked' by imposters. One, Katie, discovered that her photo was being used by a man to create a fake profile. Last year Stephen Gately, the singer, pointed out that his page on MySpace was the work of a web guerrilla and nothing to do with him.

Second LifeThe online virtual 3D world has provided rich pickings for vandals, daubing US presidential hopeful John Edwards's Second Life headquarters with Marxist slogans, pictures and virtual excrement.